Micro-channel heat exchangers, also known as flat-tube or parallel flow heat exchangers, are well known in the art, especially for automobile air conditioning systems. Such heat exchangers typically comprise an inlet manifold fluidly connected with an outlet manifold by a plurality of parallel tubes, each tube being formed to include a plurality of micro-channels. In conventional use, an airflow is passed over the surface of the heat exchanger and a refrigerant fluid is passed through the tubes and micro-channels of the heat exchanger to absorb heat from the airflow. During this heat exchange, the refrigerant fluid evaporates, while the temperature of the external airflow is lowered to levels suitable for cooling applications, such as in air conditioning units, coolers or freezers.
During operation, a refrigerant fluid flow is distributed through the inlet manifold so that each tube receives a portion of the total refrigerant fluid flow. Ideally, the fluid flow should be uniformly distributed to each of the tubes, and further each of the micro-channels therein, so as to ensure optimal efficiency in operation of the heat exchanger. However, a bi-phase refrigerant condition often exists between the inlet manifold of the heat exchanger and the tubes and micro-channels in parallel flow heat exchanger designs. That is, a two-phase fluid enters the inlet manifold of the heat exchanger and certain tubes receive more liquid-phase fluid flow while other tubes receive more gas-phase fluid flow, resulting in a stratified gas-liquid flow through the heat exchanger. This bi-phase phenomenon results in an uneven distribution of the refrigerant through the tubes and micro-channels. This, in turn, results in a significant reduction in the efficiency of the heat exchanger. Additionally, some tubes may receive more fluid flow in general than other tubes, which maldistribution also acts to hinder the efficiency of the system.
Various designs for improving the uniformity of refrigerant fluid distribution through a micro-channel heat exchanger have been developed. For example, U.S. Pat. No. 7,143,605 describes positioning a distributor tube within the inlet manifold, wherein the distributor tube comprises a plurality of substantially circular orifices disposed along the length of the distributor tube and positioned in a non-facing relationship with the inlets of respective microchannels in an effect to distribute substantially equal amounts of refrigerant to each of a plurality of flat tubes. Similarly, WO 2008/048251 describes the use of an insert inside the inlet manifold to reduce the internal volume of the inlet manifold. The insert may be a tube-in-tube design, comprising a distributor tube with a plurality of circular openings disposed along the length of the distributor tube for delivering refrigerant fluid to exchanger tubes. These designs, though showing some improvement in refrigerant distribution uniformity, still do not achieve desirable distribution uniformity and performance levels for micro-channel heat exchangers.
FIG. 1 illustrates the change in refrigerant distribution along the length of a standard distributor tube commonly used in micro-channel heat exchangers. In FIG. 1, the straight line represents an ideal distribution condition where a refrigerant fluid is evenly distributed—i.e., the refrigerant mass flow does not vary along the length of the distributor tube. The curved line in FIG. 1 represents the actual condition of refrigerant distribution. Where the curve lies below the straight line, the actual refrigerant distribution is less than ideal. Where the curve is above the straight line, the actual refrigerant distribution is too high. The actual condition curve indicates that tubes in the center of the heat exchanger receive greater fluid flow, while tubes located on the edges of the heat exchanger receive less fluid flow. The shadowed area between the two lines indicates the difference between the actual condition and the ideal condition for refrigerant distribution. The distribution uniformity for the distributor tube can be expressed by the following equation:U=(mtotal−Σ|Δm|)/mtotal where U represents the distribution uniformity of the refrigerant; mtotal represent the total amount of refrigerant flow; and Δm represents the difference between the actual amount of refrigerant flow and the ideal amount of refrigerant flow.
In view of the foregoing, there is a need for a heat exchanger design that increases uniformity of refrigerant fluid distribution and consequently increases performance levels for micro-channel heat exchangers. Accordingly, it is a general object of the present invention to provide a micro-channel heat exchanger design that overcomes the problems and drawbacks associated with refrigerant fluid flow in such parallel flow heat exchanger designs, and therefore significantly improves the uniformity of fluid distribution and overall operational efficiency.